A TDC is a circuit which converts a time component into a digital value. In a conventional digital circuit, time has been handled based on clock cycle. On the other hand, in a TDC, an element delay amount is extracted with clock timing, and thus time can be measured with reference to the delay amount of respective elements. In this manner, the time component with higher resolution than ever before can be handled as a digital value.
In general, it is difficult to realize a TDC capable of achieving both the high time resolution and the adaptation to a wide frequency range. Also, from the relation of being dependent on the delay inherent to an element, the TDC has a disadvantage of being affected by the change in delay value due to the variation in PVT (manufacturing process, power supply voltage and temperature). More specifically, the delay value ratio varies depending on frequency due to the change in delay value. As a result, the time resolution of the TDC becomes nonconstant, and it becomes necessary to make a correction according to situations or to prepare a number of elements with a certain level of margin.